Design flaws
It was mildly addictive at first, but as someone pointed out, the guns may be fun and interesting because you can use any of them to the end... but then there is no incentive to upgrade really because all the guns are "good" and I'd argue the weapon you start with is actually one of the better ones.
Biggest design flaw for me though that made me stop playing was the level challenges. The ones that say "Only use this weapon" are VERY frustrating if you took the meteor or drill perks because if one of your meteor or drills kills an enemy (which is very hard and annoying trying to make sure YOU get the KO shot), even if you never change weapons you don't get completion on the challenge.
Good basic idea and was fun for a while, but needs some work.